I use baking soda to simulate rust on burned and schorched metal. Paint the part(s) in your general rust colour, then quickly, while the paint is fully wet (It works better if you apply the base coat with a brush) dip the part in baking soda. It will provide a very convincing texturized effect. Airbrush another shade of rust, then wash and drybrush with darker and lighter shades, respectively. For the inside of the tank, flat black all around, and one key thing:
If the hole is large enough, have one or two loose wires hanging inside the damaged hole. I've found little else that is as effective a representation of a damaged interior.